Audio frequency amplifier



L. D. NORTON Oct. 11, 1938.

AUDIO FREQUENCY AMPLIFIER Filed Nov. 20, 1934 INVENTOR Lela/21d DJV'm-on IIIIJ ATTORNEYS Patented Oct. 11, 1938 I Iteland DQNorton, Bridgeport, Oonns lassig-norito This invention relates to f audio-frequencyamplifiers and more particularly to means for automatically controlling-the gain of the amplifier tortionless-means for effecting widevariation of' the amplification factor at one or-more stages of a multistage audio-amplifier in response to slight changes above a predetermined power-level in the output-of a later or final stage of said amplifier.

Thermionic used in transmission systems receiving =voice frequency currents from telephone lines on directly from microphones, 'in order to provide sufficient energy for recording the voice frequencies upon .phonographrecords- In' order that such record be relatively free from; distortion and to prevent blastingw-hen the voice currents rise above a :desired amplitude; it has been foundnecessary to control thepower .gain of the amplifiersoas to compensate for variations in the power level of the input-. circuit feeding to the amplifier. Ii automatic-control means are to .be employed,-it becomes essential to insure that the rate. of vari ation of the amplification factor of thethermionic devices be' kept well below the'lowest'audiofrequencies within the range of the recording mechanisms. It is accordingly a further object of this invention-toprovi'de an automatic volume control havinga time constant relatively large as compared with the lowest audio-frequency to be recorded.

Other objects of my invention w-ill be pointed outin the 7 following description taken in conjunction with the drawing; the-:single figure of which illustrates one embodiment of my inven .tion as applied to an audio-frequency: amplifier of special design connected inductively to an input circuit. and also connected inductively to an output circuit, the latter including suitable phonographic recording devices." I The invention accordingly consists in the features of construction, combinationsof elements, and arrangements of parts as will be exemplified in the system'tos'be-hereinafterdescribed and the scope of the application of which will be indicated in the following claim. Referring to the drawing, the amplifier, indicated generally by.A will be seen to be of the three-stage typewith a transformer input .to,

thefirst stage, which comprises a variable mu electron tube. V-'l of the type; 58.". The second stage includes a tubev V-2 of the same type which is resistance coupled to theoutput. of the amplifiers have heretofore been Dictaphone :GorporationfNew' York, N. Y.,-a 'corporationof NewIYork Application November:20,11934flserial Ne. 753,845 l e 1 claim; 1 -(o1 ;"'i'9 u1) first "stage. amplifier, which is transformer coupled to the secondstage, comprises two electron power tubes -V-'3V-4 of the type-45 arranged in push-pull relation. #The output of the push-pull stage is transformer coupled to an output circuit con taining phonographic recording devices. A control tube V--'5 of the type 56 is coupled to the output circuit in shunt with the recording devicesand to the grid circuits of the tubes V-l-- :V-Z. The'power supply for the amplifier comprises a power transformer T, a full wave rectiher a filter F and and a voltage divider R.

A signal input circuit l includes the primary coil l of an audio-frequency transformer 5. The input circuit 1 may be suitably connected to a telephonetransmission line by 0.1 mfd. blocking "condensers CI andC-2, to a microphone'circuit, or to any other circuit suitable for impression of signal voltages at voice frequencies upon the transformer primary 4.

, "Tube V| comprises a plate 'Lfa cathode 8, a

filament 9 for heating the cathode 8, a control grid iii, a screen grid H, and a suppressor grid 42. In -the present arrangement, as it has been found desirable to decrease the amplification factor of tube Vl', the suppressor grid l2 of that tube is connected to the plate 1. The input circuit of the tube V--! includesthe secondary coil t3 "of the transformer -5 which is connected.

to the control grid H] by the conductor M. A gridresistor R-l of about 250,000 ohms is .connected across the coil [3. Coil I3 is also connected by means of a resistor R-l (0.1-meg.)

and connection H3 at a point I! with a line IB, connected by means of a slider contact 19 with aresi'stanceelement R-l5 (50.0 ohms) of the voltage divider R. The cathode v8 .of the tube LV- -l is connected at 21 to a conductor 22, connectedat 2.3 to the voltage divider R,

positive end of the voltage divider is indicated at' 25 and the negative end at 26.. The control grid I0 is normally negatively biased with respect tothe cathode 8 by the voltage drop across the portion of the resistance element it-I5 extendingto the right of the slider l9.

The filament of tube V-I as well as those of tubes V:-2 and V--5 are connected, as indicated by the conductors .XX; i .a secondary coil 43 of the power transformer T, the primary 28 of which receives energy from a 110420 which at that point is groundedas indicated at 24. The

The third and final stage of the i terminals of the voltage divider by the rectifier E. A further secondary coil 45 supplies current to the filaments of the tubes V-3--V4 dicated by the conductors Y-Y.'

Returning to the description of the amplifier,

the plate 1 of tube V-l is connected through the coupling resistor R-3 (0.1 meg.) .toa conductor 41 connected at 25 to the positive'end of the voltage divider R. The plate circuit return is made through the cathode 8 to the point 23 of the voltage divider. Thus the plate circuit of tube V'l includes resistance element R-l8 (20,000 ohms) of the voltage divider. The screen grid H of the tube VI is connected by means of a conductor 49 and resistor R-4 (25,000 ohms) toa slider 56 which has movable contact with the element Rl6 of the voltage divider. Thus the potential of the screengrid may be varied as desired. The screen grid to cathode circuit is shunted by a condenser C-.4

(0.5 mid.) and a resistor R-2 (0.1 meg.) which serve as a filter for that circuit. A similar filter comprises a condenser C9 (2. mid.) shunted across the control grid to cathode circuit of tube V-! and the resistor R-l.

The plate i of tube V-l is connected to the control grid 54 of the tube V--2 through a blocking condenser C-I2 (0.006 mid). The plate 57 of tube V-2 is connected through the primary coil 58 of an audio-frequency transformer 59 through a suitable resistor R9 (75,000 ohms) to the conductor 41. The cathode GI of tube V-2 is connected as at B2 to conductor 22. The plate to cathode circuit of tube V-2 is, therefore, connected across the same resistance element of the voltage divider as the similar circuit of tube V--I. As with tube V-l the suppressor grid 63 is connected to the plate 51. Since the second amplifier stage is transformer coupled to the push-pull stage the tube V.2 need not have as high an amplification factor as tube VI. It is desirable, however, that it have greater distortionless power capacity. Hence the screen, grid 64 vis connected directly to the suppressor grid 63'and plate 51. The control grid circuit of tube V-2 includes the resistors R. 5 (250,000 ohms) and R'6 (.1 meg.) and is connected through the conductors l6 and I8 to the slider contact [9 and the voltage divider. Thus the control grid bias for tube V2 is provided by the voltage drop across the same portion of resistance element Rl5 that provides grid bias for tube Vl. A condenser C-'l (0.5 mfd.) provides an audio-frequency path between transformer primary and cathode 6|. A condenser C5 (0.5 mfd.) provides a by-pass between cathode 6| and grid resistor R5.

The amplified audio-frequency voltages on the plate 57 are impressed upon the grids 10 and H of tubes V3 and V-4, the'impressed voltage being derived from the secondary coil 14 of transformer 59. The return circuit for the two grids 10 and H is by means of a wire 15 connected to the midpoint of transformer coil 14 and leading to the negative side 26 of voltage divider R. The filament cathodes l6 and 11 of these tubes are connected through the lines as inand short telephone transmission lines.

Y-Y directly to the terminalsof the secondary coil 45 of power transformer T. The midpoint of this coil is connected through a. biasing resistor R-I1 (1,000 ohms) to the negative side of the rectifier to voltage divider circuit at a point 19. Thus the amount of negative bias at the grids of the push-pull amplifier tubes is determined by the resistor R-ll. The plates and 8| of amplifier tubes V3 and V-4 are connected together through the primary coil 82 of the output audio-frequency transformer 83. The midpoint of this coil is connected through a resistor R-l8 (1,000 ohms) to a point 85 in the positive side of the power rectifier circuit between the choke coils 38 and'39.

The resistor Rl8 is selected to provide a sufliciently high voltage in the plate to cathode circuit of the push-pull tubes and since the plate to cathode circuits of this stage of the amplifier do not include resistances of the voltage divider there is small chance for feed-back from the output stage to the earlier stages of the amplifier. The secondary coil 86 of transformer 83 is connected by means of an output circuit,'ineluding a condenser C8 (0.5 mfd.) in its high potential side, to oneor more phonographic recording instruments as indicated at 81 and 88. The low potential side of this output circuit is grounded-as at 85.

When the input circuit I receives voice currents from a microphone, telephone circuit, or otherwise, each amplifier stage will increase the energy of such voice currents and if the amplifier is properly designed sufiicient en'ergy will be delivered to the recording instruments 81, 88 to enable them to out sound grooves upon the records used in connection therewith. Unfortunately, the power level of the input circuit I may vary, for example, from +10 to 40 db., rating 0 at 0.006 watt. Therefore, it is not possible always to get satisfactory recording results with an amplifier of this type unless compensation is made for such variations in input power level. Such variations are particularly likely to occur where the amplifier is connected alternately to long Now suppose the input circuit power level to be 40 db. and that there is an input circuit coupling loss of 20 db. Then, if for satisfactory recording results it is desirable to hold the power level in the output circuit at +20 db., it follows thatthe amplifier must have a maximum power ain of at least 80 db. T

But should the energy level of the'input circuit rise to 20 db. it would follow that the'80 db. gain of the amplifier would result in a too high power level in the output circuit.

- From such a'condition there would arise a lack of uniformity in the amplitude of, the recording, causing the recordsto vary greatly in loudness when reproduced; and even in respect to a single record, great differences in loudness'would occur at different points in the record. 1

To obviate this difiiculty the present amplifier has been provided with an automatic volume control comprising an electron tube V5 of the type 56. The "cathode 90 of this tube is connected at point 9| to the voltage'divider. The control grid 92 is connected by means of a slider contact 93 with the resistance element R l2 (2,000 ohms) of the voltage divider through the grid resistor R-l0 (0.1 meg). The grid 92 is also connected by awire 96 through a condenser C3 (0.1 mfd.) at a point 98 with the high potential side of the output or load circuit. The

plate 99 of tube V- is connected to the conductor l8 and thus to slider contact i9 and the voltage divider. A resistor Rll (0.1 meg.) is included in the conductor l8 at a point between its connection with the plate 99 and the slider IS. The plate circuit of the tube V-5 thus includes resistor RF| l, resistance element R-I3 (7,000 ohms) and the lefthand portion of R-l5, while the grid to cathode circuit of the said tube includes that portion of the voltage divider resistance Rl2 that is disposed between the slider contact 93 and the point 9 I. It will be seen that the resistor Rll is included not only in the plate circuit of the control tube V5 but also in the grid circuits of the variable mu amplifier tubes Vl and V--2. This resistance provides the means for varying the grid bias of the variable mu tubes in response to variations in the energy level of the output circuit impressed on the grid of V-5.

Normally, in operation, the slider contact 93 is adjusted so as to provide a negative potential on the grid 92 of the control tube of such value that the control tube is biased approximately to cut-off. The biasing of grid 92 by means of the slider contact 93 is adjusted so that until the energy of the output circuit exceeds a predetermined level a negligible quantity of current will flow in the plate circuit of the control tube V5. When, however, the said output energy rises even slightly above its predetermined level the grid 92 of the control tube will become more positive and current will flow through the plate circuit thereof and through the resistance-R-l l. The voltage drop across the resistance R--H will cause the control grids l0 and 54 of the variable mu tubes V-l and V2 to become more negative and thus reduce the amplification factors of these tubes. By proper designing, the control tube may be made to produce great variation in the amplification factors of the variable mu tubes in response to very slight increases in the power level of the output circuit. As soon as the decreased amplification of the tubes Vl and V2 effects sufficient reduction in the output level, current of any appreciable value will cease to flow through the resistor R l I and the grids of the tubes VI and V-2, becoming again more positive, the amplification factors of those tubes will increase in value. In this way a substantially constant power level may be maintained at the output stage of the amplifier irrespective of the intensity of the voice currents received at 'the input stage thereof.

It should here be noted that the automatic control of the amplification factors of variable mu tubes in a plurality of different stages of the amplifier presents a problem quite different from that where control is confined to a single stage. The maximum amount of amplification allowed .to the tubes of each stage becomes an important factor. Since as in the present embodiment of the invention the grids of the two controlled tubes are connected to the same control element and receive their normal voltage bias from the same source, it is highly important that proper means be employed to prevent feed-back between the respective grid circuits. This purpose is attained herein by shunting a condenser C-lll (1. mid.) across the grid cathode circuits of the controlled tubes. This condenser serves also to determine the time constant of the control circuit which is substantially equal to the product of the series resistance R-ll and the shunt capacitance C-I0. By properly determining the value of capacitance C--l0 in relation to resistance R-ll any desired time constant may be attained. Practically this is adjusted so that there will be sufficient delay to prevent a rapid shift from low tohigh amplification and vice versa between syllables or words of the matter recorded.

To obtain the best results tubes V-l and V2 are operated at ground potential. The grounding of their cathodes at 24 tends to stabilize the circuits of these tubes. The grid circuit of the control tube is also grounded through a condenser CI3 (4 mid).

Another important feature of the present amplifier is the resistor R'l, which tends to sta bilize the grid circuit of the first amplifier tube Vl.

It will be seen that there is provided a construction of an essentially practical nature in which the several objects of this invention are attained.

As many possible embodiments may be made of the above invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

In an audio-frequency amplifying system of the type wherein the vacuum tube amplifier derives its operating potentials from a potentiometer connected across a direct current source of power, a vacuum tube control circuit deriving its necessary operating potentials from said potentiometer and adapted to cause a grid bias of said amplifier to become more negative as the output of said amplifier increases, comprising, in combination, a control vacuum tube having a cathode, anode and grid, a capacitative connection from said grid to said amplifier at a point which varies in potential in accordance with the audio-frequency output of said amplifier, another connection from said grid through a first resistance to an adjustable tap on said potentiometer at a first point near the negative end thereof, said first point being effectively grounded through a first condenser, a connection from said potentiometer at a second point intermediate its ends to ground and to a cathode of said amplifier, a connection from said control tube cathode to said potentiometer at a third point intermediate said first and second points, a connection from said control tube anode to a grid circuit of said amplifier and through a second resistance to an adjustable tap at a fourth point on said potentiometer intermediate said second and third points, a second condenser connected between said anode and said second point, the values of said second resistance and said second condenser determining the time constant of operation of said control circuit, and other necessary operating circuits of said amplifier deriving their potentials from said potentiometer at points between the positive end thereof and said second point.

LELAND D. NORTON. 

